1. Technical Field
This invention is in the field of photographic film, and in particular relates to opaque backing layers which are coated on a photographic support. More particularly the invention relates to dispersions of carbon black and polymer for use in preparing an opaque backing layer which can be removed in the fluids used during the processing step in which the photographic image is fixed.
2. Discussion of the Prior Art
The addition of so-called "backing layers" to one side of a film support containing a photographic silver halide layer coated on the other side of the support, is well-known in the art. These layers are applied for various reasons: as anti-halation layers, anti-friction layers, anti-static layers and protective layers, for example. Carbon black may be used as the opacifying agent for such purposes, e.g., in anti-halation layers, as described in U.S. Pat. No. 2,271,234, or for providing an opaque backing layer for diffusion transfer type photographic elements in which a reusable negative can be obtained, as explained in U.S. Pat. No. 3,677,790. In U.S. Pat. No. 4,211,837, Blake et al., removable opaque backing layers for photographic silver halide elements are described in which the opaque backing layer contains carbon black, a polymeric binder, and a scavenging agent such as a silver halide or silver nitrate, deactivating for impurities in the carbon black. The carbon black is dispersed in a binder, with conventional additives such as surfactants and coating aids, and the resulting dispersion or slurry is stored until needed. The problem with this is that such dispersions are not storage-stable for more than a few days, after which phase separation takes place.
It is an object of this invention to provide a dispersion of carbon black in polymer which is storage-stable and suitable for coating as a high density opaque backing layer.
It is a further object to provide a stable carbon black dispersion of improved coatability which is especially useful for forming a high density removable opaque backing layer on a photographic diffusion transfer element.